In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. For example, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful to prepare lithographic printing plates typically comprise one or more imageable layers applied over the hydrophilic surface of a substrate. The imageable layers include one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the element is considered as positive-working. Conversely, if the non-imaged regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
New UV laser, visible laser, or thermal imaging has become increasingly important in the printing industry because of their stability to ambient light. The imageable elements used for the preparation of lithographic printing plates have been designed to be sensitive to near-UV radiation, visible radiation, or heat or infrared radiation and can be exposed using laser diodes, thermal heads of more usually, infrared laser diodes that image in response to signals from a digital copy of the image in a computer a platesetter. This “computer-to-plate” technology has generally replaced the former technology where masking films were used to image the elements.
These imaging techniques often require the use of water or a developer (neutral to alkaline pH) to remove exposed (positive-working) or non-exposed (negative-working) regions of the imaged layer(s). In some instances, the negative-working lithographic printing plate precursors are designed for development on-press using a suitable fountain solution, lithographic printing ink, or both.
The negative-working precursor can have a water-soluble or water-dispersible overcoat (also sometimes known as an “oxygen impermeable topcoat” or “oxygen barrier layer”) disposed over the negative-working radiation-sensitive imageable layer. The topcoat can be the outermost layer. Such overcoat layers can comprise one or more water-soluble poly(vinyl alcohol)s. Details about such overcoats are provided in WO 99/06890 (Pappas et al.) and U.S. Pat. No. 5,998,095 (Nagase).
EP Publication 2,275,258A2 (Oohashi et al.) describes lithographic printing plate precursors having overcoat layers that comprise a modified poly(vinyl alcohol) having alkylene oxide or amino groups. Anionic modified poly(vinyl alcohol) are described in overcoat layers in JP Publication 2007-245,495 (Endo).
As noted above, imaged lithographic printing plate precursors are processed or developed in some manner to remove appropriate regions of the imageable layers. In the case of negative-working precursors that generally include a protective overcoat, the non-imaged (non-exposed) regions are removed by processing. In most uses, the protective overcoats are removed by a separate water wash step followed by a developing step using a separate developer bath to remove the non-image areas of the imaged layer. In general, processing is carried out using specific processing solutions that are designed to remove or solubilize specific imaging chemistry. If the processing solution is not designed properly for a specific imaged precursor, the presence of a water-soluble protective overcoat having high oxygen barrier efficiency (for example using a polymer comprising vinyl alcohol units) causes the formation of undesirable sludge formation of sludge or debris in the processor tank and rollers occurs when the imaged precursor is processed without a separate wash step before development, especially as the developer loading increases.
As a result, material can be deposited on non-image regions of the lithographic printing plate resulting in difficulties in printing start up or dirty printed impressions. Furthermore, the length of a loading cycle is low because it is limited by the poor solubility of the protective overcoat material, and the cleaning efforts in the processor are increased if the protective overcoat is not washed off in a separate wash step.
There is a need for an improved negative-working lithographic printing plate precursor that has a protective overcoat having a high oxygen barrier efficiency (low oxygen permeability results in good latent image keeping), high solubility of the overcoat material in the processing solutions, good scratch resistance, and good protective overcoat adhesion to the underlying imageable layer. It is known that poly(vinyl alcohol) (PVA) has low oxygen permeability especially if it has high degree of saponification (that is, low amount of vinyl acetate residues). Highly saponified PVA is known to have very low water-solubility. The solubility of highly saponified PVA in processing solution that are suited for imageable layers is even worse than its solubility in water because of its high content of salts, surfactants, solvents, and other components. There is a need to solve this problem for negative-working lithographic printing plate precursors.